Phased array antennas in general must be calibrated at several levels in order to operate effectively. Since a phased array typically uses many active and passive components to form its aperture vector (amplitude and phase) state, these must be aligned in order to form a high efficiency radiation beam. While various conventional techniques are available to provide the initial calibration state, to correct for variability (recalibration), and to identify failed components for replacement, calibration often can be verified by or controlled through either near field or far field sensing methods. Very large microwave and RF phased array radars have apertures that can have lengths exceeding over 1000 wavelengths, for example, in any aperture dimension, making either near or far field sensing either expensive, very difficult, or both.
In one known technique, there is calibration of the individual components in the chain extending from the antenna beam port. After calibration in the factory, these components are assembled in the field to produce an initially calibrated aperture. However, this method of calibration has a number of drawbacks, including for example, heavy reliance on a bookkeeping system to catalogue the calibrated components in the beam formation chain, such that mistakes have the potential to produce an un-calibrated system. This is a concern with very large arrays requiring the calibration of greater than 10,000 independent paths in order to achieve array calibration. In addition, this method by itself does not verify the calibrated state, such that whenever a vector state error is produced, which can be due to assembly error, natural component degradation, or error, it cannot be directly detected by this method alone.
The above methods rely on near or far field sensors to detect the aperture vector state, both initially, and during the life of the antenna system. Large phased array planar systems impose significant problems for such calibration approaches because of the reliance on near or far field sensors. In the antenna near field, sensors may require large and accurate equipment to position a probe antenna in a plane near to and parallel with the fielded antenna aperture plane. The size and accuracy of such equipment can be significant cost factors. Beyond this, such a near field sensor may require considerable system downtime in order to determine the antenna aperture vector state, which is a prerequisite for calibration and recalibration. Far field calibration sensors alone are a complex and expensive approach for large array calibration because of the significant separation distances involved.
The calibration methods typically used for phased arrays are clearly disadvantageous for large apertures. In addition, however, a considerable challenge comes in the form of a digital beam forming system. As a result, additional microwave connectivity is needed to calibrate the digital channels. Without such calibration, these digital networks may drift, and as a result, produce array and system losses and errors that can degrade the system performance and capabilities.
It will be appreciated that various known RF injection techniques have been used in moderately-sized phased array antennas. These include beacon methods, such as the use of aperture peripheral horn antennas, or near field antennas, or the use of array radiating element mutual coupling. However, these methods have significant disadvantages when considered for large phased arrays. For example, the beacon methods require a source antenna in the large array near field, thus causing difficulties in providing physical support while remaining outside of the large array field of view, source physical stability, and limitations on the large array aperture field variation, particularly when frequency scanned elements are used. Mutual coupling methods are also limited when these elements are used, largely because inter-element coupling magnitude may be insufficient for accurate calibration.